Class-E radio frequency amplifier for use with an implantable medical device

ABSTRACT

A medical apparatus includes an extracorporeal power source that transmits electrical power via a radio frequency signal to a medical device implanted inside an animal. The extracorporeal power source has a Class-E amplifier with a choke and a semiconductor switch connected in series between a source of a supply voltage and circuit ground. An output node of the amplifier is formed between choke and the switch and connected to a transmitter antenna. A shunt capacitor couples the amplifier&#39;s output node to the circuit ground. Controlled operation of the switch produces bursts of the radio frequency signal that are pulse width modulated to control the amount of energy being sent to the implanted medical device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Patent ApplicationNo. 60/776,853 filed Feb. 24, 2006.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to implantable medical devices, whichdeliver energy to stimulate tissue for the purposes of providing therapyto the tissue of an animal, and more particularly to an radio frequencyamplifier for use in such a medical device.

2. Description of the Related Art

A remedy for a patient with a physiological ailment is to implant anelectrical stimulation device that provides provide therapy to thepatient. An electrical stimulation device is a small electronicapparatus that stimulates an organ or part of an organ with electricalpulses. It includes a pulse generator, implanted in the patient, andfrom which electrical leads extend to electrodes placed adjacent tospecific regions of the organ.

An improved apparatus for physiological stimulation of a tissue includesa wireless radio frequency (RF) receiver implanted as part of atransvascular platform that comprises at least one stent-electrode thatis connected to the wireless RF receiver and an electronic capsulecontaining a stimulation circuitry. The stimulation circuitry receivesthe radio frequency signal and, from the energy of that signal, derivesan electrical voltage for powering the implanted device. The electricalvoltage is applied in the form of suitable waveforms to the electrodes,thereby stimulating the tissue of the organ.

The radio frequency (RF) signal generation is a significant part of theelectrical stimulation apparatus and it usually involves the use of anRF amplifier. The RF amplifier of choice typically has been a Class-A orClass-AB amplifier in those cases where linearity is of utmost concern.The class of an analog amplifier defines what proportion of the inputsignal cycle is used to actually switch on the amplifying device. AClass-A amplifier is switched on 100% of the time. A Class-AB amplifieruses a signal cycle that is greater than 50%, but less than 100% toswitch on the amplifying device. Unfortunately, these amplifiers are notvery efficient and dissipate a significant amount of energy. Theefficiency of a power amplifier is defined as the ratio of output powerand input power expressed as a percentage.

Recently, a different kind of amplifier, known as a switching amplifier,has been developed. A particularly useful switching amplifier is calleda Class-E amplifier. Switching amplifiers have relatively high powerefficiency due to the fact that perfect switching operation does notdissipate power. An ideal switch has zero impedance when closed andinfinite impedance when open, implying that there is zero voltage acrossthe switch when it conducts current (on state) and zero a non-zerovoltage across it in the non-conductive state (off state). Consequently,the product of voltage and current (power loss) is zero at any time.Therefore, a Class-E amplifier has a theoretical efficiency of 100%,assuming ideal switching.

From a theoretical standpoint, a Class-E amplifier can provide veryefficient RF amplification. However, in practice, Class-E amplifiers donot achieve anywhere close to the theoretical limits. Some embodimentsof the prior art techniques use a relaxation oscillator to drive theamplifier. With this technique, it is impossible to control the range ofthe power depending on the need. In other embodiments, a regulator isused to control the power feed. In this case, heat is generated in thecontrol system itself and the amplifier's efficiency is subsequentlylowered. Therefore, there is a need to improve the performance ofpractical Class-E RF power amplifiers based on the fundamentalunderstanding of the loss generation processes. An optimal design canmake the heat dissipation so low such that heat-sink are not required.

SUMMARY OF THE INVENTION

The present invention provides a Class-E RF power amplifier suitable tobe used in an extracorporeal power source that supplies a medical deviceimplanted inside an animal.

The extracorporeal power source comprises a power transmitter thatproduces a radio frequency signal and an antenna that is connected tothe power transmitter. The power transmitter includes a Class-Eamplifier that has a choke and a switch connected in series between asource of a supply voltage and circuit ground. An amplifier output nodeis formed between choke and the switch and is connected to the antenna.A shunt capacitor couples the amplifier output node to the circuitground.

The medical device receives the radio frequency signal transmitted fromthe antenna and derives an electrical voltage from energy of the radiofrequency signal in order to power components of the medical device.

In a preferred embodiment, the switch comprises a semiconductor device,such as a MOSFET. Ideally the semiconductor device has a feedbackcapacitance that is less than 10% of its input capacitance. It also ispreferred that channel resistance and a peak current rating of thesemiconductor device are such that the arithmetic product of the channelresistance and the peak current rating is less than 3% of the supplyvoltage.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of a wireless transvascular platform, thatincludes external and internal components, for stimulating tissue insidea patient;

FIG. 2 is a schematic diagram of an exemplary implanted medical devicewith an external component containing a Class-E RF amplifier;

FIG. 3 is a detailed schematic diagram of the Class-E RF amplifier; and

FIG. 4 depicts waveforms for signals in the Class-E RF amplifier.

DETAILED DESCRIPTION OF THE INVENTION

With initial reference to FIG. 1, a wireless transvascular platform 10for tissue stimulation includes an extracorporeal power source 14 and amedical device 12 implanted inside the body 11 of an animal. Theextracorporeal power source 14 includes a battery that powers atransmitter that sends a first radio frequency (RF) signal 26 to themedical device 12. The medical device 12 derives electrical power fromthe energy of the first radio frequency signal 26 uses that power toenergize and electronic circuit 30 mounted on an electronic carrier 31.The first radio frequency signal 26 also carries commands to configurethe operation of the medical device.

A second RF signal 28 enables the medical device 12 to transmitoperational data back to the extracorporeal power source 14. Such datamay include physiological conditions of the animal, status of themedical device and trending logs, for example, that have been collectedby the implanted electronic circuit 30 and sent via the second radiofrequency signal 28. This data is provided transmitted by theextracorporeal power source 14 monitoring equipment so that medicalpersonnel can review the data or be alerted when a particular conditionexists.

The implanted medical device 12 includes the electronic circuit 30mentioned above which has an RF transceiver and a tissue stimulationcircuit, similar to that used in conventional pacemakers anddefibrillators. That electronic circuit 30 is located in a large bloodvessel 32, such as the inferior vena cava (IVC), for example. One ormore, electrical leads 33 and 34 extend from the electronic circuit 30through the animal's blood vasculature to locations in the heart 36where pacing and sensing are desired. Each lead has an electricalconductor enclosed in an electrically insulating outer layer. Theelectrical leads 33 and 34 terminate at electrode assemblies 38 at thoselocations.

With reference to FIG. 2, the internal components comprise an implantedmedical device 12 includes a stimulation circuit 132 having a firstreceive antenna 152 within the antenna assembly 124 in which the antennais tuned to pick-up a first RF signal 26 at a first radio frequency F1.The first receive antenna 152 is coupled to a data detector 156 thatrecovers data and commands carried by the first RF signal 26. That dataspecifies operational parameters of the medical device 12, such as theduration that a stimulation pulse is applied to the electrodes 120 and121. The recovered data is sent to a control circuit 155 for thatmedical device, which stores the operational parameters for use incontrolling operation of a pacing signal generator 158 that appliestissue stimulating voltage pulses across the electrodes 120 and 121.

The control circuit 155 also is connected to a pair of sensor electrodes157 that detect electrical activity of the heart and provideconventional electrocardiogram signals which are utilized to determinewhen cardiac pacing should occur. Additional sensors for otherphysiological characteristics, such as temperature, blood pressure orblood flow, may be provided and connected to the control circuit 155.The control circuit stores a histogram of pacing, data related to usageof the medical device, and other information which can be communicatedto the extracorporeal power source 14 or another form of a datagathering device that is external to the patient.

The first receive antenna 152 also is connected to a rectifier 150 thatextracts energy from the received first RF signal. That energy is usedto charge a storage capacitor 154 that supplies electrical power to thecomponents of the implanted medical device 12. Specifically, the radiofrequency, first RF signal 26 is rectified to produce a DC voltage (VDC)that is applied across the storage capacitor 154.

The DC voltage produced by the rectifier 150 also is applied to afeedback signal generator 160 comprising a voltage detector 162 and avoltage controlled, first radio frequency oscillator 164. The voltagedetector 162 senses and compares the DC voltage to a nominal voltagelevel desired for powering the medical device 12. The result of thatcomparison is a control voltage that indicates the relationship of theactual DC voltage derived from the received first RF signal 26 and thenominal voltage level. The control voltage is fed to the control inputof the voltage controlled, first radio frequency oscillator 164 whichproduces an output signal at a radio frequency that varies as a functionof the control voltage. For example, the first radio frequencyoscillator 164 has a center, or second frequency F2 from which theactual output frequency varies in proportion to the polarity andmagnitude of the control signal and thus deviation of the actual DCvoltage from the nominal voltage. For example, the first radio frequencyoscillator 164 has a first frequency of 100 MHz and varies 100 kHz pervolt of the control voltage with the polarity of the control voltagedetermining whether the oscillator frequency decreases or increases fromthe second frequency F2. For this exemplary oscillator, if the nominalvoltage level is five volts and the output of the rectifier 150 is fourvolts, or one volt less than nominal, the output of the voltagecontrolled, first radio frequency oscillator 164 is 99.900 MHz (100MHz-100 kHz). That output is applied to via a first data modulator 165to a first transmit antenna 166 of the implanted medical device 12,which thereby emits a second RF signal 28. Data regarding physiologicalconditions of the animal and the status of the medical device 12 aresent from the control circuit 155 to the first data modulator 165 whichamplitude modulates the second RF signal 28 with that data.

As noted previously, the electrical energy for powering the medicaldevice 12 is derived from the first RF signal sent by the extracorporealpower source 14. The extracorporeal power source 14 uses power from arechargeable battery 170 to periodically transmit pulses of the first RFsignal 26. The first RF signal 26 is pulse width modulated to vary themagnitude of energy received by the implanted medical device 12. Thepulse width modulation is manipulated to control the amount of energythe medical device receives to ensure that it is sufficiently poweredwithout wasting energy from the battery 170 in the extracorporeal powersource 14. Alternatively, the first RF signal 26 can also be modulatedby amplitude modulation to vary the magnitude of energy received by theimplanted medical device 12.

To control the energy of the first RF signal 26, the extracorporealpower source 14 contains a second receive antenna 174 that picks up thesecond RF signal 28 from the implanted medical device 12. Amplitudemodulated data is extracted from the second RF signal 28 by a datareceiver 116 and sent to the controller 106. Because the second RFsignal 28 also indicates the level of energy received by medical device12, this enables extracorporeal power source 14 to determine whethermedical device should receive more or less energy. The second RF signal28 is sent from the second receive antenna 174 to a feedback controller175 which comprises a frequency shift detector 176 and aproportional-integral (PI) controller 180. The second RF signal 28 isapplied to the frequency shift detector 176 which also receives areference signal at the second frequency F2 from a second radiofrequency oscillator 178. The frequency shift detector 176 compares thefrequency of the received second RF signal 28 to the second frequency F2and produces a deviation signal AF indicating a direction and an amount,if any, that the frequency of the second RF signal has been shifted fromthe second frequency F2. As described previously, the voltagecontrolled, first radio frequency oscillator 164, in the medical device12, shifts the frequency of the second RF signal 28 by an amount thatindicates the voltage from rectifier 150 and thus the level of energyderived from the first RF signal 26 for powering the implanted medicaldevice 12.

The deviation signal ΔF is applied to the input of theproportional-integral controller 180 which applies a transfer functiongiven by the expression GAIN/(1+s_(i)·τ), where the GAIN is a timeindependent constant gain factor of the feedback loop,

is a time coefficient in the LaPlace domain, and s_(i) is the LaPlaceterm containing the external frequency applied to the system. The outputof the proportional-integral controller 180 on line 181 is an errorsignal indicating an amount that the voltage (VDC) derived by theimplanted medical device 12 from the first RF signal 26 deviates fromthe nominal voltage level. That error signal corresponds to anarithmetic difference between a setpoint frequency and the product of atime independent constant gain factor, and the time integral of thedeviation signal.

The error signal is sent to the control input of a pulse width modulator(PWM) 182 which forms an amplitude modulator within a power transmitter173 and produces at output signal that is on-off modulated as directedby the error input. The output from the pulse width modulator 182 is fedto a second data modulator 184 which modulates the signal with data fromthe controller 106 for the medical device 12. The second data modulator184 feeds the RF signal to a Class-E type RF power amplifier 186 fromwhich the signal is applied to a second transmit antenna 188.

In addition to transmitting electrical energy to the implanted medicaldevice 12, the extracorporeal power source 14 transmits operationalparameters which configure the functionality of the medical device. Theimplanted medical device 12 also sends operational data to theextracorporeal power supply. A data input device, such as a personalcomputer 100, enables a physician or other medical personnel to specifyoperating parameters for the implanted medical device 12. Such operatingparameters may define the duration of each stimulation pulse, aninterval between atrial and ventricular pacing, and thresholds forinitiating pacing. The data defining those operating parameters aretransferred to the extracorporeal power source 14 via a connector 102connected to the input of a serial data interface 104. The data receivedby the serial data interface 104 can be applied to a microprocessorbased controller 106 or stored directly in a memory 108.

FIG. 3 illustrates a unique Class-E amplifier 300 that is employed asthe RF power amplifier 186. The modifications comprise an overratedswitch with low channel resistance and feedback capacitance, a drivecircuit closely integrated with the switch, a mechanism to tunecomponents by adjusting the drive frequency, and an oscillator the dutycycle of which is controlled by non-linearly manipulating a sinusoidaldrive signal.

The Class-E amplifier 300 is operated by a voltage or current of theoutput signal from the second data modulator 184, which is passedthrough an input matching network 355 in which the mixed modulatorsignal is AC coupled to a fraction of the sine wave signal and the baseline is shifted by a suitable design parameter. The waveform of thedrive signal at the output of the second data modulator 184 is depictedin FIG. 4. The drive signal is formed by pulses of the first radiofrequency F1 that are present during the on time of the amplifier 300wherein the pulse duty cycle is determined by the signal on line 181from the proportional-integral controller 180. The period that theamplifier is on is given by Ts_(ON)=η₁Tf, where Tf is the total time ofon and off periods that form one signal cycle, and η₁ is the ratio of ontime and the total time. Note that Tf=1/F1. These higher frequencypulses provide finer control of the drive signal without affecting thefirst radio frequency F1, as occurred with prior methods. Note that thisunique pulse design also makes the design more robust and relativelyimmune to load variations. Thus it allows tuning of components by slightadjustment of drive frequency and control of the output power of theamplifier.

The Class-E RF power amplifier 300 has a supply input connected to asource of a supply voltage V_(E) and coupled to ground by an inputcapacitor 310. A choke 320 couples the supply voltage V_(E) to theswitch 325. The choke 320 maintains the current that flows through theswitch 325 during its on time, such that after the switch opens, thecurrent flow is distributed between a shunt capacitor 330 and a resonanttank circuit 335, that includes the second transmit antenna 188. Theratio of this distribution is a function of the phase of the periodiccycle of the resonant tank circuit 335 and of the timings of the switch325. For maximum efficiency, the switch 325 should close while thevoltage across the shunt capacitor 330 is substantially to zero.

The switch 325 is a low impedance device, preferably a MOSFET. It isimportant to over specify the switch 325 by preferably an order ofmagnitude or more. For example, if the maximum expected current is oneampere, the switch should be rated to handle a transient current of upto ten amperes. The switch element has a low channel resistance and lowfeedback capacitance. The channel resistance preferably should be suchthat the arithmetic product of channel resistance and the peak currentrating of the switch is less than 3% of the supply voltage V_(E) to theamplifier 300. The feedback capacitance preferably should be such thatit is less than 10% of the input circuit capacitance. The drive circuitis closely integrated with the switch 325 wherein the circuit boardlayout is chosen based on the selected component configuration, forexample by mounting the components as close together as possible. Inaddition, the loop containing the peak current is spatially located inclose proximity to the switch 325.

The tank circuit 335 couples an amplifier output node 340, that islocated between the choke 320 and the switch 325, to ground. The tankcircuit 335 approximates the resonant waveform that is measurable in aninductively coupled load, as is represented by the “body tissue coupledload” 380. The majority of the coupling with the body tissue isinductive L_(COUPLING) and losses associated with that coupling arerepresented by R_(LOAD).

To maintain the oscillatory condition, it is desirable to have eitherpredictable phase and gain parameters or control over these parameters.When a load is presented, the drive is increased to meet a predefinedsetpoint, or a variable setpoint, alternatively a combination of thesetwo methods. In one implementation, it is sufficient to provide a startcondition that initially closes the switch 325 for a limited period oftime, followed by providing feedback such that the switch is turned offwhen sufficient current is detected through the tank circuit.

In addition to the power level feedback provided by the implantedmedical device 12, it is also possible to provide further feedbackcontrol by sampling the output power level at the second transmitantenna 188. One technique for controlling the energy of the first radiofrequency signal 26 uses a lower frequency pulse width modulationmethod. Here, the average output power is sampled and the amplifier ispulse width modulated at a frequency that is one or more orders ofmagnitude lower than the first radio frequency F1. In one example, thePWM frequency could be 200 kHz for a 20 MHz Class E amplifier.

In this feedback version, the drive circuit varies the on-time (or dutycycle) of the switch 325 in response to the output of the powertransmitter 173 as measured by a pickup coil 370 coupled to the secondtransmit antenna 188. The voltage induced across the pickup coil 370 isrectified and filtered by an RC network 375 to provide a feedbackvoltage that is translated pulse width modulator 182 to a duty cycle ofthe drive signal, wherein a greater feedback voltage translates to alower duty cycle, and a lesser feedback voltage translates to a higherduty cycle. Thus the duty cycle is proportional to the measurement fromthe pickup coil 370.

The feedback circuit measures the field level generated under load andproportions the drive (on-duration of the amplifier switch 325)accordingly to maintain the oscillatory condition. The feedback circuitmay not be self starting. However, it could be operated as a modifiedself oscillating circuit, in which there is a first radio frequency F1operated at a minimum idle current. A unique feature of the presentinvention is the use of a sinusoidal envelope voltage that isnon-linearly manipulated to derive the rectangular pulses. This enablesthe number of components in the Class-E amplifier to be reducedsubstantially.

For linear applications, the PWM frequency must be selected inconformity with the maximum bandwidth and phase linearity desired in thefiltered output signal. For example, the maximum frequency componentsmust be at least one half of the PWM frequency, but may need to be lowerdepending on the maximum allowed phase variance, which is caused by thedigitization process.

The foregoing description was primarily directed to a preferredembodiment of the invention. Although some attention was given tovarious alternatives within the scope of the invention, it isanticipated that one skilled in the art will likely realize additionalalternatives that are now apparent from disclosure of embodiments of theinvention. For example, the present invention was described in thecontext of a device for cardiac stimulation, but can be employed withother types of implanted medical device systems. Accordingly, the scopeof the invention should be determined from the following claims and notlimited by the above disclosure.

1. A medical apparatus comprising: an extracorporeal power source havinga power transmitter that produces a radio frequency signal and anantenna that is connected to the power transmitter, the powertransmitter including a source of the radio frequency signal, a Class-Eamplifier that comprises a switch; the power source comprising a pulsewidth modulator that receives the radio frequency signal and a controlsignal and in response thereto produces an output signal that is appliedto a control input of the switch; an output sensor which provides afeedback signal indicating an intensity of radio frequency signalapplied to the antenna, and wherein the pulse width modulator respondsto the feedback signal by varying the output signal; and a medicaldevice for implantation inside an animal, the medical device receivingthe radio frequency signal transmitted from the antenna and deriving anelectrical voltage from energy of the radio frequency signal whichelectrical voltage powers components of the medical device.
 2. Themedical apparatus as recited in claim 1 wherein the Class-E amplifierfurther comprises a choke connected in series with the switch between asource of a supply voltage and circuit ground, with an amplifier outputnode being formed between the choke and the switch and connected to theantenna, and a shunt capacitor coupling the amplifier output node to thecircuit ground.
 3. The medical apparatus as recited in claim 1 whereinthe switch is selected from a group consisting of a semiconductor deviceand a MOSFET.
 4. The medical apparatus as recited in claim 2 wherein theswitch is a semiconductor device that is connected to a load and theradio frequency signal is proportional to an induced voltage at theload.
 5. The medical apparatus as recited in claim 1 wherein the outputsignal controls a duty cycle of the switch.
 6. The medical apparatus asrecited in claim 5 wherein the switch is a MOSFET that has a channelresistance and a peak current rating, wherein an arithmetic product ofthe channel resistance and a peak current rating is less than 3% of asupply voltage to the Class-E amplifier.
 7. The medical apparatus asrecited in claim 1 wherein the switch is rated to conduct a transientcurrent level that is at least ten times a maximum level of a currentthat the switch is expected to conduct.
 8. The medical apparatus asrecited in claim 1 wherein the pulse width modulator modulates the radiofrequency signal with the control signal to produce the output signal.9. The medical apparatus as recited in claim 1 wherein the output signalhas an envelope defined by the control signal.
 10. The medical apparatusas recited in claim 1 wherein the output signal comprises pulses of theradio frequency signal, wherein each pulse has a shape that is definedby the control signal.